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VFDs are not really all that susceptible to voltage imbalances, all they are doing is rectifying the AC to DC. A severe imbalance can affect the lifespan of the internal DC bus capacitors because they end up working harder to smooth out the resultant ripple on the DC bus.

But if the PC boards that are frying are not the boards inside of the VFDs, then this has nothing to do with the VFDs. PC boards never run off of line voltage so there are power supplies somewhere providing them with their operating voltage. If the boards are failing because of line problems, those would have to be the CRAPPIEST power supplies on the market! Most modern power supplies are now built to handle a really wide range of input voltage. The ones I use for 480V supplies will accept anything from 260 to 550VAC input.

One thing that would be more likely to me is that the control boards are picking up stray voltages on their analog and/or digital input wiring and they have inadequate filtering to protect against that. This can happen especially when someone ran control wiring with unshielded cables in parallel with power cables. That may have been the HVAC contractor or it may even be an internal wiring issue in their units.

So most likely this is a QC issue with their equipment or installation instructions and they are throwing out this “voltage imbalance” as a red herring to distract their customers from the real issue.
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